The present invention relates to a clutch of an automatic transmission, and more particularly, to a return apparatus of a clutch piston of an automatic transmission.
A multi-plate clutch is structured having a plurality of alternating clutch plates and disks, and is used to transmit power in motorcycles and automatic transmissions of vehicles. The conventional multi-plate clutch assembly, with reference to FIG. 5, includes a plurality of clutch plates 104, which are mounted substantially in parallel within slots of an inner slotted portion 102 of a clutch retainer 100, and a plurality of clutch disks 110, which are mounted between the clutch plates 104 within slots of an outer slotted portion 108 of a hub 106. That is, the clutch plates 104 and the clutch disks 110 are inserted partially within the slots of the inner slotted portion 102 of the clutch retainer 100 and the outer slotted portion 108 of the hub 106, respectively.
A piston 112 is mounted in an area defined by the clutch retainer 100, and a hydraulic pressure chamber 114 is formed between the piston 112 and the clutch retainer 100. A return spring 116, which is a compression spring, is provided on a side of the piston 112 opposite to that where the hydraulic pressure chamber 114 is formed to provide a constant biasing force to the piston 112 in a direction toward the hydraulic pressure chamber 114.
With the supply of a sufficient hydraulic pressure to the hydraulic pressure chamber 114, the piston 112 overcomes an elastic force of the return spring 116 such that the piston 112 moves in a direction toward the clutch plates 104 and the clutch disks 110 to compress the same, thereby engaging the multi-plate clutch to allow for the transmission of power. If the hydraulic pressure is then exhausted from the hydraulic pressure chamber 114, the piston 112 moves in the opposite direction as a result of the biasing force of the return spring 116. This removes the force applied to the clutch plates 104 and the clutch disks 110 by the piston 112 so that the transmission of power is discontinued.
However, the exhaust of hydraulic fluid is often incomplete. That is, although the hydraulic pressure in the path between a hydraulic pressure control apparatus and the hydraulic pressure chamber 114 is fully exhausted, part of the fluid in the hydraulic pressure chamber 114 is not exhausted as a result of a centrifugal force generated by the rotation of the clutch, thereby generating a centrifugal force hydraulic pressure. In the case where the clutch is rotating at a high speed, the centrifugal force hydraulic pressure may maintain engagement of the clutch.
FIG. 5 shows two types of clutch return mechanisms according to the prior art, one of which is, as shown in an upper-half part, a balance piston type, and the other of which is, as shown in a lower-half part, a check ball type.
According to the balance piston type, a balance piston 118 is mounted inwardly from the piston 112 as shown in the upper part of the example of a conventional multi-clutch assembly of FIG. 5, and the return spring 116 is interposed between the piston 112 and the balance piston 118. Also, an opposing hydraulic pressure is supplied to a space between the piston 112 and the balance piston 118 such that the opposing hydraulic pressure enables the piston 112 to return.
According to the check ball type, with reference to the lower part of the example of a conventional multi-clutch assembly of FIG. 5, a check ball 120 is mounted at an outer extremity portion of the piston 112, with this area of the piston 112 and the adjacent member being formed in such a manner as to allow operation of the check ball 120. Various forces act on the check ball 120 including the force of the hydraulic pressure, centrifugal force resulting from the rotation of the multi-plate clutch, and a buoyancy force provided by the fluid used to create hydraulic pressure. If a rotational speed of the clutch is increased, the centrifugal force acting on the check ball 120 overcomes the centrifugal force hydraulic pressure to create the flow of fluid, that is, to enable exhaust of the remaining fluid.
However, in the case of the first example above in which the balance piston 118 is used, to effectively offset the centrifugal force hydraulic pressure, an inner diameter of the balance piston 118 must be smaller than an inner diameter of the piston 112. As a result, the size of the transmission is increased.
Further, with respect to the application of the check ball 120, it is difficult to design the check ball 120 and maintain the quality of the same to meet the precise design parameters. It is also difficult to fabricate the check ball 120 to the precise design parameters, as the surfaces of the check ball 120 and its contacting surface must be fabricated with exactitude.
According to the present invention there is provided a return apparatus for a clutch piston of an automatic transmission which includes a return spring, the elastic force of which increases as the rotational speed of the clutch piston increases such that the clutch piston is prevented from engaging when hydraulic pressure is released. The return apparatus according to the invention further includes a spring support rotating at a same speed as the clutch piston, with the return spring abutted at an end against the spring support and exerting an elastic force on the clutch piston, the elastic force increasing as the clutch piston rotational speed increases.
The return spring of the invention preferably includes a plane spring plate curved convexly at its central portion, in a radial direction with respect to the rotation axis of the clutch piston, and a mass unit disposed at the curved central portion, where the return spring is abutted against the spring support at an end of the plane spring plate. The opposite end of the plane spring plate is connected to the clutch piston. The mass unit is, preferably, cylindrically shaped.
The return spring may also comprise a central portion having a predetermined length and mass, and a pair of elastic fins extending in a radial direction with respect to the rotation axis of the clutch piston, at a predetermined angle on opposite sides of the central portion. Also a distal end of one of the elastic fins is preferably abutted against the spring support and a distal end of the other elastic fin is abutted against the clutch piston.
In a further aspect of the invention a return spring for a clutch piston of an automatic transmission comprises a v-shaped body with a force generating mass disposed at the apex of the body. The v-shaped body has two resilient arms configured to bear in opposition against a spring support and the clutch piston. The force generating mass disposed at the apex is configured and dimensioned to exert a spreading force on the resilient arms in response to rotation of the clutch piston sufficient to overcome any centrifugal force hydraulic pressure acting on the piston. In a preferred embodiment, the force generating mass is formed integrally with the v-shaped body.
In another alternative embodiment of the invention, a clutch piston system for an automatic transmission includes a clutch piston acting on at least one clutch plate in response to applied hydraulic pressure, and centrifugal force means for returning the clutch piston to a non-acting position in response to discontinued hydraulic pressure. In a preferred embodiment, the centrifugal force means comprises support means rotating with the clutch piston and centrifugal force biasing means bearing against the support means. The centrifugal force biasing means acts on the clutch piston with increasing force proportional to rotational speed of the clutch piston. In one preferred embodiment, the centrifugal force biasing means comprises a v-shaped body and a force generating mass. The v-shaped body defines an apex and two resilient arms. The force generating mass is disposed at the apex and is configured and dimensioned to exert a spreading force on the resilient arms in response to rotation of the clutch piston sufficient to overcome centrifugal force hydraulic pressure acting on said piston.